Ventricular assist devices are receiving ever-increasing attention in our society where 400,000 Americans are diagnosed with congestive heart failure each year (Rutan, P. M., Galvin, E. A.: Adult and pediatric ventricular heart failure, in Quall, S. H. (ed), Cardiac Mechanical Assistance Beyond Balloon Pumping, St. Louis, Mosby, 1993, pp. 3-24). As a result, collaborative efforts among health care professionals have focussed on the development of various systems to assist the failing heart. These comprise both extracorporeal and implantable pulsatile ventricular assist devices (VAD), as well as non-pulsatile assist pumps.
Extracorporeal systems include the Pierce-Donachy VAD and the Abiomed BVS-5000 VAD. The Pierce-Donachy VAD is positioned on the patient's abdomen and propels blood by means of a pneumatically actuated diaphragm. Its use as a bridge to transplant is well-documented (Pae, W. E., Rosenberg, G., Donachy, J. H., et al.: Mechanical circulatory assistance for postoperative cardiogenic shock: A three-year experience. ASAIO Trans 26:256-260, 1980; Pennington, D. G., Kanter, K. R., McBride, L. R., et al.: Seven years' experience with the Pierce-Donachy ventricular assist device. J Thorac Cardiovasc Surg 96:901-911, 1988). The Abiomed BVS-5000, also an extracorporeal device, is fixed vertically at the patient's bedside and is attached to the heart with percutaneous cannulae that exit the patient's chest below the costal margin (Champsaur, G., Ninet, J., Vigneron, M., et al.: Use of the Abiomed BVS System 5000 as a bridge to cardiac transplantation. J Thorac Cardiovasc Surg 100:122-128, 1990).
The most frequently used implantable systems for clinical application include the Novacor VAD (Novacor Division, Baxter Health Care Corp.) and the Heartmate (Thermocardiosystems) (Rowles, J. R., Mortimer, B. J., Olsen, D. B.: Ventricular Assist and Total Artificial Heart Devices for Clinical Use in 1993. ASAIO J 39:840-855, 1993). The Novacor uses a solenoid-driven spring to actuate a dual pusher plate. The pusher plate compresses a polyurethane-lined chamber which causes ejection of blood (Portner, P. M., Jassawalla, J. S., Chen, H., et al: A new dual pusher-plate left heart assist blood pump. Artif Organs (Suppl) 3:361-365, 1979). Likewise, the Heartmate consists of a polyurethane lined chamber surrounded by a pusher plate assembly, but a pneumatic system is used to actuate the pusher plate (Dasse, K. A., Chipman, S. D., Sherman, C. N., et al.: Clinical experience with textured blood contacting surfaces in ventricular assist devices. ASAIO Trans 33:418-425, 1987).
Efficacy of both the extracorporeal and implantable pulsatile systems has been shown (Rowles, J. R., Mortimer, B. J., Olsen, D. B.: Ventricular Assist and Total Artificial Heart Devices for Clinical Use in 1993. ASAIO J 39:840-855, 1993). However, certain complications are associated with the use of extracorporeal systems, including relatively lengthy surgical implantation procedures and limited patient mobility. The use of totally implantable systems raises concerns such as high cost of the device, complex device design, and again, relatively difficult insertion techniques.
Centrifugal pump VADs offer several advantages over their pulsatile counterparts. They are much less costly; they rely on less complicated operating principles; and, in general, they require less involved surgical implantation procedures since, in some applications, cardiopulmonary bypass (CPB) is not required. Thus, an implantable centrifugal pump may be a better alternative to currently available extracorporeal VADs for short- or medium-term assist (1-6 months). In addition, the use of centrifugal pumps in medium-term applications (1-6 months) may allow the more complex, expensive VADs, namely the Novacor and the Heartmate, to be used in longer term applications where higher cost, increased device complexity, and involved surgical procedures may be justified.
Prior art relating to centrifugal blood pumps is Canadian Patent No. 1078255 to Reich; U.S. Pat. No. 4,927,407 to Dorman; U.S. Pat. No. 3,608,088 to Dorman; U.S. Pat. No. 4,135,253 to Reich; Development of the Baylor-Nikkiso centrifugal pump with a purging system for circulatory support, Naifo, K., Miyazoe, Y., Aizawa, T., Mizuguchi, K., Tasai, K., Ohara, Y., Orime, Y., Glueck, J., Takatani, S., Noon, G. P., and Nose', Y., Artif. Organs, 1993; 17:614-618; A compact centrifugal pump for cardiopulmonary bypass, Sasaki, T., Jikuya, T., Aizawa, T., Shiono, M., Sakuma, I., Takatani, S., Glueck, J., Noon, G. P., Nose', Y., and Debakey, M. E., Artif. Organs 1992;16:592-598; Development of a Compact Centrifugal Pump with Purging System for Circulatory Support; Four Month Survival with an Implanted Centrifugal Ventricular Assist Device, A. H. Goldstein, MD; U.S. patent application titled "Radial Drive for Implantable Centrifugal Cardiac Assist Pump", University of Minnesota; Baylor Multipurpose Circulatory Support System for Short-to-Long Term Use, Shiono et al., ASAIO Journal 1992, M301.
Currently, centrifugal pumps are not implantable and are used clinically only for CPB. Examples include the Biomedicus and the Sarns centrifugal pumps. The Biomedicus pump consists of an impeller comprised of stacked parallel cones. A constrained vortex is created upon rotation of the impeller with an output blood flow proportional to pump rotational speed (Lynch, M. F., Paterson, D., Baxter, V.: Centrifugal blood pumping for open-heart surgery. Minn Med 61:536, 1978). The Sarns pump consists of a vaned impeller. Rotation of the impeller causes flow to be drawn through the inlet port of the pump and discharged via the pump outlet port (Joyce, L. D., Kiser, J. C., Eales, F., et al.: Experience with the Sarns centrifugal pump as a ventricular assist device. ASAIO Trans 36:M619-M623, 1990). Because of the interface between the spinning impeller shaft and the blood seal, several problems exist with both these pumps, including excessive wear at this interface, thrombus formation, and blood seepage into the motor causing eventual pump failure (Sharp, M. K.: An orbiting scroll blood pump without valves or rotating seals. ASAIO J 40:41-48, 1994; Ohara, Y., Makihiko, K., Orime, Y., et al.: An ultimate, compact, seal-less centrifugal ventricular assist device: baylor C-Gyro pump. Artif Organs 18:17-24, 1994).
The AB-180 is another type of centrifugal blood pump that is designed to assist blood circulation in patients who suffer heart failure. As illustrated in FIG. 1, the pump consists of seven primary components: a lower housing 1, a stator 2, a rotor 3, a journal 4, a seal 5, an impeller 6, and an upper housing 7. The components are manufactured by various vendors. The fabrication is performed at Allegheny-Singer Research Institute in Pittsburgh, Pa.
The rotor 3 is in the lower housing 1 and its post protrudes through a hole in the journal 4. The impeller 6 pumps blood in the upper housing 7 and is threaded into and rotates with the rotor 3. The impeller shaft passes through a rubber seal 5 disposed between the upper housing 7 and the journal 4, rotor and stator assembly. The upper housing 7 is threaded into the lower housing 1 and it compresses the outer edge of a rubber seal 5 to create a blood contacting chamber. In this manner, blood does not contact the rotor 3, journal 4, or lower housing 1. The upper housing 7 is connected to an inlet and outlet flow tubes 8, 9, called cannulae, that are connected to the patient's circulatory system, such as between the left atrium, LA, and the descending thoracic aorta, DTA, respectively. Through this connection, blood can be drawn from the left atrium, LA, through the pump, and out to the aorta, DTA.
The impeller 6 spins by means of the rotor 3 and stator 2 which make up a DC brushless motor. The base of the rotor 3 has four magnets that make up two north-south pole pairs which are positioned 90 degrees apart. The stator 2 is positioned around the rotor 3 on the lower housing 1. The stator 2 comprises three phases. When it is energized, it creates a magnetic force that counteracts the magnets in the rotor 3 causing the rotor 3 and impeller 6 to spin, as is well known with brushless DC motors.
A peristaltic pump infuses lubricating fluid into a port of the lower housing to lubricate the spinning rotor. The fluid prevents contact between any solid internal pump components during pump activation. It forms a layer of approximately 0.001 inches around the rotor and the impeller shaft. This fluid bearing essentially allows wear-free operation of the pump. The fluid passes around the rotor and flows up along the rotor post. Eventually, it passes out through the rubber seal 5 and into the upper housing 7 at the impeller shaft/seal interface. Fluid does not escape through the outer periphery of the housing seal because the upper housing is tightened down and sealed with a rubber O-ring to prevent leakage.
The spinning impeller 6 within the top housing 7 causes fluid to be drawn from the inlet flow tube 8 toward the eye of the impeller. The impeller 6 then thrusts the fluid out to the periphery of the upper housing 7. At this point, the fluid is pushed through the outlet tube 9 by centrifugal force. The pump typically consumes 3-5 Watts of input power to perform the hydraulic work necessary to attain significant physiologic benefits.
The prior art AB-180 pump has certain drawbacks which limit its efficacy as a cardiac assist device. The present invention describes several discoveries and novel constructions and methods which vastly improve such a pump's operation.